Recently, electrophotographic technology has been widely applied to the field of printers, as well as the field of copiers, due to its immediacy and formation of high-quality images. Photoreceptors lie in the core technology of electrophotography, and organic photoreceptors using organic photoconductive materials have been developed, since they have advantages such as non-pollution and ease in production in comparison with inorganic photoconductive materials. In general, an organic photoreceptor is composed of an electroconductive support and a photosensitive layer disposed thereon. Photoreceptors are classified into a so-called single-layer photoreceptor having a single photosensitive layer containing a binder resin dissolving or dispersing a photoconductive material therein; and a so-called multilayered photoreceptor composed of a plurality of laminated layers including a charge-generating layer containing a charge-generating material and a charge-transporting layer containing a charge-transporting material.
In the organic photoreceptor, changes in use environment of the photoreceptor or changes in electric characteristics during repeated use may cause various defects in an image formed with the photoreceptor. In a reliable method for forming a good image, an undercoat layer containing a binder resin and titanium oxide particles is provided between an electroconductive substrate and a photosensitive layer (for example, refer to Patent Document 1).
The layer of the organic photoreceptor is generally formed by applying and drying a coating liquid prepared by dissolving or dispersing a material in a solvent, because of its high productivity. In such a case, since the titanium oxide particles and the binder resin are incompatible with each other in the undercoat layer, the coating liquid for forming the undercoat layer is provided in the form of a dispersion of titanium oxide particles.
Such a coating liquid has generally been produced by wet-dispersing titanium oxide particles in an organic solvent using a known mechanical pulverizer, such as a ball mill, a sand grind mill, a planetary mill, or a roll mill, by spending a long period of time (for example, refer to Patent Document 1). Furthermore, it is disclosed that when titanium oxide particles are dispersed in a coating liquid for forming an undercoat layer using a dispersion medium, an electrophotographic photoreceptor that exhibits excellent characteristics in repeated charging-exposure cycles even under conditions of low temperature and low humidity can be provided using titania or zirconia as the dispersion medium (for example, refer to Patent Document 2). However, since higher-quality images are required, the performance of the conventional technology is insufficient in various respects such as quality of images formed and stability of coating liquids during a manufacturing process.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. HEI 11-202519
[Patent Document 2] Japanese Unexamined Patent Application Publication No. HEI 6-273962